1. Field of the Invention
The present invention relates to a winding-type common-mode choke coil for removing a common-mode noise on a transmission line.
2. Description of the Related Art
Techniques disclosed in Japanese Unexamined Patent Application Publication No. 2003-168611 and Japanese Unexamined Patent Application Publication No. 2000-133522 are currently available for a common-mode choke coil.
The common-mode choke coil includes a core and two wires wound in a coil around a core section of the core. The core has flange sections at both ends thereof. The two ends of each of the coils are connected respectively to electrodes of the flange sections. A ferrite plate straddles the top sides of the flange sections.
Such a structure of the common-mode choke coil removes the common-code noise entering a differential transmission line or the like.
The known above-described common-mode choke coil has the following problem.
Before being sold as commercial products, the common-mode choke coils are typically subjected to an immunity test. In the immunity test, the coils are exposed to electromagnetic interferences likely to occur thereto in order to determine whether the coils withstand a variety of electromagnetic interferences.
In the immunity test of the common-choke coil to the common-mode noise, the common-mode coil as a test specimen is arranged at the front end of a receiver IC (Integrated Circuit) connected to a transmitter IC connected via a differential transmission line. A differential signal is transmitted from the transmitter IC to the receiver IC via the differential transmission line, and a common noise is induced in the differential transmission line to be superimposed on the differential signal. In this condition, the transmitter IC and the receiver IC are checked for any malfunction. Such an immunity test is referred to as an induction immunity test.
Since inductance of the common-mode choke coil as a test specimen and input capacitance of the receiver IC form a resonator circuit in the above-described known common-choke coil during the induction immunity test, the effectiveness of control of the common-mode noise is lowered at a resonance frequency of the resonator circuit and in a frequency band in the vicinity of the resonance frequency. In such a case, the transmitter IC and the receiver IC malfunction, and the test specimen may not pass the induction immunity test.